Batteries store or release electric charge that is generated from electrochemical reactions. An example of such a device would be a lithium-ion battery cell, which has three active elements: the anode, the cathode and the electrolyte. The cell also has an important electrically neutral fourth component, a separator between the anode and the cathode. For example, separators for conventional, planar lithium-ion batteries are typically solid micro-porous polyolefin films that are assembled in a sheet form and rolled in the form of a cathode/separator/anode/separator stack. This stack is rolled tightly and inserted into a can, filled with electrolyte, and then sealed. For example, reference to P. Arora and Z. Zhang, “Battery separators,” Chem. Rev., 2004, 104, 4419-4462, may help to illustrate the state of the art in battery separators, and is therefore incorporated by reference as non-essential subject matter herein.
FIG. 1 shows a cross sectional view of a conventional planar lithium-ion battery cell. The battery cell 10 has a cathode current collector layer 11 on top of which a cathode layer 12 is assembled. The cathode layer 12 is covered by a separator layer 13 over which an assembly of the anode layer 14 and the anode current collector layer 15 are placed. Note that the anode layer 14, the cathode layer 12, and the separator layer 13 are parallel. The cell 10 is then filled with an electrolyte that resides in the pores in the electrodes and the separator that serves as the transport medium for ionic movement between the anode layer 14 and the cathode layer 12. In a lithium ion battery, the electrolyte typically includes lithium. The current collectors 11, 15 are used to collect the electrical energy generated by the battery cell 10 and connect it either to other cells or to external devices. An external device can be electrically powered by the battery or provide electrical energy to the battery during recharging.
The separator serves as the mechanical barrier between the two electrodes to prevent them from shorting, at the same time allowing for ionic transport through the electrolyte in the pores. Separators need to have good mechanical integrity and chemical inertness. They should also have well defined and consistent porosity and tortuosity in order to uniformly transport the ions between the electrodes. A variety of materials have been used as separators in batteries. Some examples of separator/battery combinations currently being used are: cellulose materials for alkaline and silver-zinc batteries; polyethylene-silica composite material for flooded lead acid batteries; glass fibers for recombinant lead acid batteries; hydrophilized polypropylene for nickel-metal hydride batteries; and polyolefin porous separators in lithium-ion batteries. Other separators that have been used in different battery systems have included rubber, nylon, cellophane etc.
However, conventional separators that are made with polymers used in batteries suffer from some or all of the following limitations. In order to maintain mechanical integrity, the minimum functional separator thickness has to be significantly higher when using a polymeric or cellulose material than when using a more rigid material. This is generally undesirable since the separator does not take part in the electrochemical reaction, so that increasing its thickness degrades the battery's gravimetric and volumetric energy density. Also, polymeric separators may not have adequate resistance to puncture by metallic and particulate contamination. Improving the material toughness of a separator may significantly improve the safety of the battery from contamination related shorting during manufacture and also during battery operation. Polymeric separators may also have insufficient chemical and electrochemical stability in the battery environment. For example, some organic materials such as polyethylene oxide, when used in lithium ion batteries, undergo degradation at the high cathodic potentials. The separator degradation inherently limits the potentials to which the cathode can be charged to and thus limits the capacity one can achieve in lithium-ion batteries.
Thus, there is a need in the art for an improved separator for use in batteries that may give the battery longer life, improved stability or safety, and/or higher energy densities.